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The invention relates generally to the field of optical communication, particularly to generating and switching extremely narrow pulses. Such pulses are used generally for ultra-fast optical communication, especially for Dense Time Division Multiplexing (DTDM), Code Division Multiplexing (CDM), and Demultiplexing.
In optical communication networks there is a continuous demand for increasing the transmitted information (vocal, video and data) rate and capacity. Optical fibers used in optical networks have a great capability to transmit the required optical-information at high-rate and large-volume. On the other hand, there are several components in optical networks that limit the capability of the optical fibers to transmit high-rate and large-volume information and thus cause to bottlenecks in the optical networks. Such components are the optical modulators and the electronic switches. The optical modulators are limited in their capability to produce optical pulses (logical digital bits) at the desired width and speed and the electronic switches are limited in their capability to switch and route the optical information at the desired rate.
Two methods are used to increase the transmission rate in the optical networks. The first one is the Wavelength Division Multiplexing (WDM) or its improvement Dense Wavelength Division Multiplexing (DWDM). The second method is the Time Division Multiplexing (TDM).
WDM or DWDM methods increase the transmission rate by using parallel information channels. The information in each optical channel is carried by a different light frequency. These light frequencies are very accurate and well defined and are produced by Distributed Feedback Lasers (DFB) or by Distributed Brugg Reflector lasers (DBR). In Brugg reflection multiple reflections, usually received from a stack of multiple dielectric layers or from a grating along its plane, are all in phase. The light frequencies of the channels are combined together and are inserted into the input of a single optical fiber, which carries their information to its output. The combined light frequencies at the output of the fiber are received by the DWDM network, which separates them back to different parallel channels that each has a specific light frequency.
For example, if the bit rate at each information channel is at a frequency f and xcex1 channels are inserted to a single optical fiber, then the bit rate F at the fiber is xcex1xc2x7f. At the other side of the fiber after the demultiplexing of the DWDM, the bit frequency of each channel is reduced back to f. A typical number for xcex1 is between 80 to 160 for WDM and up to 320 for DWDM. The number of channels used is limited for the following reasons:
1. The optical fiber has a limited bandwidth (F when it has good optical properties, such as low loss and low chromatic dispersion.
2. The light frequency separation between the different channels is F/xcex1. Increasing the number of channels xcex1 decreases the light frequency separation F/xcex1 between the channels. The light frequency separation F/xcex1 between the channels must be larger than the line width of the DFB or the DBR lasers which serve as light sources of the channels. Due to the limitation of the line width of these lasers, it is clear that the number of channels J is also limited. Currently the number of channels used (320) is reaching its maximum value.
While the WDM and the DWDM methods increase the information rate in the optical fibers by using parallel information channels, the TDM method is used to increase the bit rate in each channel. According to this method, the bits of several parallel channels having the same light frequency are interleaved in a predetermined periodic order to create a single serial data stream of a single channel. This method is very effective when using a buffer, which accumulates and compresses the data of several channels into a dense serial data stream of a single channel by reorganizing this data with suitable delays. However the bit rate of this method and others is still limited by the bit rate and duty cycle that the light sources (DFB and DBR lasers) can produce.
There are two techniques to produce light pulses which are used as, logical bits.
The first technique is to modulate the current injected to the DFB and DBR lasers. This technique, called direct modulation suffers from the following disadvantages:
1. It has a relatively low duty factor (repetition rate) due to long recovery time of the lasers.
2. It produces chromatic dispersions in the optical fibers due to broadening of the emitted spectral line of the modulated lasers. This is caused by spontaneous emission, jittering, and shifting of the gain curve of the lasers during the current injection.
Due to the above disadvantages, an alternative way or second technique of modulation is used. In this method the lasers are operated in a Continuous Wave (CW) mode. Separate modulators do the modulation of the radiation beams. These modulators are usually made from interference devices such as Mach-Zender""s, directional couplers and active half wave-plates combined with polarizers and analyzers.
Applying modulating voltage that affects the electro-optical characteristics of the materials from which the modulators are made activates these devices. The electro-optical properties of these devices are used to produce phase shifts and polarization change. Such changes involve with creating and removing space charges, which change the density of the charge carriers within these electro-optic materials. The formation rate of the space charges is mainly dependent upon the speed and the magnitude of the applied voltage and can be in the order of sub nanoseconds. The charge removal is usually slower and is mainly dependent upon the relaxation time of these materials (lifetime of charge carriers) and can be relatively long. Thus the time-on of the modulation is much longer than the time-off of these modulators. Accordingly, the width of the pulses and the duty cycle of the modulation in this technique are mainly dependent upon the long off time of the modulators.
Today the highest bit rate that can be achieved is about 10 G bits per channel and is limited by the modulation rate of the modulators, the pulse width that they produce, and the switching time of the electronic switches. This bit rate is not sufficient even for the present needs and the demand for a capability to produce and rout faster bit rate will increase in the near future.
Accordingly, one object of the invention is to provide a method and means for increasing the transmitted information rate and capacity for voice, video, and data.
Other objects are to provide:
an optical communication system for performing four cooperative functions to enhance data transmission speed: produce very narrow optical pulses in different parallel channels, multiplex the optical pulses of the channels into a single serial bit stream having a very high bit rate, transmit the optical pulse information at a very high bit rate, and demultiplex, at a very high speed, the serial pulse stream back into the original information channels;
an all-optical modulator for converting optical pulses into extremely narrow optical pulses for using in optical communication networks;
an all-optical modulator for converting optical pulses into extremely narrow optical pulses for using in Dense Time Division Multiplexing (DTDM) optical communication networks;
all-optical communication system including modulators in several different optical channels for performing three cooperative functions to enhance data transmission density and speed: converting optical pulses into extremely narrow optical pulses in different parallel channels, interleaving the optical pulses of the different channels into one dense serial optical stream of pulses to produce DTDM, and transmitting the pulses, at a very high bit rate, along an optical fiber;
an all-optical modulator for converting optical pulses into extremely narrow optical pulses;
such an an all-optical modulator that is activated by a control beam;
a self-triggered all-optical switch for converting, by demultiplexing, a high rate stream of serial optical pulses, at the output of a single optical fiber, into multiple information channels;
an all-optical switch that is activated by a control beam for converting, by demultiplexing, a high rate stream of serial optical pulses, at the output of a single optical fiber, into multiple information channels;
a self-triggered all-optical switch for converting, by demultiplexing, a high rate serial of interleaved optical pulses, at the output of a single optical fiber, into multiple information channels; and
all-optical communication system including modulators, particularly all-optical modulators that are activated by control beams or self-triggered modulators for performing four cooperative functions to increase data transmission speed: converting optical pulses, at several different optical channels, into extremely narrow pulses, encoding the narrow pulses to form codes, interleaving the coded optical pulses of the channels into one dense optical stream of codes to produce DTDM, and transmitting the codes in an optical fiber at a very high bit rate;
Further objects are as follows:
to provide an all-optical communication system including switches, particularly those that are activated by control beams or self-triggered all-optical switches for performing two cooperative functions to enhance the data transmission rate;
to be able to receive, from the output of a single optical fiber, a stream of high rate coded optical pulses interleaved from multiple parallel information channels;
to be able to convert, by demultiplexing, the serial stream of the codes, into multiple information channels according to a predetermined code which dictates to which of the information channels the pulses will be routed; and
to be able to demultiplex, by Code Division deMultiplexing (CDM), a series of coded pulses of interleaved channels into multiple parallel information channels.
Yet further objects and advantages will become apparent from the ensuing description and appended drawings.
An optical system for modulating, switching, multiplexing, demultiplexing, routing and routing packets of data. The system has two versions: it can be used either as an all-optical modulator or as an all-optical switch. For both versions, the system can be operated either as a self-triggered system or it can be activated by at least one control beam. When the system is self-triggered, it includes at least one input, at least one interference device, and at least one output. The input is arranged to receive input radiation pulses for directing each of the input radiation pulses as multiple radiation pulses that propagate along multiple radiation paths having lengths. The interference device is arranged to receive, from the multiple radiation paths, multiple radiation pulses. The interference device receives the multiple radiation pulses at timings corresponding to the lengths of the radiation paths; The interference device produces and directs, toward the output, one interference pattern out of a group of multiple interference patterns that are producible by the interference device according to the timings. The output is arranged to receive, from the interference device, the interference pattern. The output selectively emits output radiation pulses according to the timings in which the interference device receives the radiation pulses.